Process for sub-microsecond time transfer using weak gps/gnss signals

ABSTRACT

Sub-microsecond time transfer in a GPS/GNSS receiver using a weak GPS/GNSS signal is provided. The digitized complex baseband signal and the generated PN code are cross-correlated for each code period so as to output a complex correlation value at each code epoch of the generated PN code, where a sequence of the output correlation values form a data stream representing the navigation message. Bit synchronization generates bit sync pulses at bit boundaries. The location of a target segment having a known sequence at a known bit location in the navigation message is detected by searching through a plurality of subframes and accumulating search results for the plurality of subframes. Transmission time of the target segment is determined from the detected location of the target segment, with a certain time ambiguity. Accurate local time is determined by solving the time ambiguity using approximate time obtained from an external source.

CROSS REFERENCE TO RELATED APPLICATION

This Application is a Continuation of application Ser. No. 12/747,505filed Jun. 10, 2010, which is a National Phase of PCT/US2008/086613filed Dec. 12, 2008, which claims benefit of U.S. ProvisionalApplication No. 61/007,711 filed Dec. 14, 2007, which are allincorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to GPS/GNSS receivers. More specifically,the present invention relates to time transfer in GPS/GNSS receiversusing weak GPS/GNSS signals.

The use of GPS/GNSS signals for time transfer at the sub-microsecondlevel is a well-established art. However, most conventional methodsdepend on the ability of a GPS/GNSS receiver to accurately demodulatetiming information from the received GPS/GNSS signal. This implies thatthe signal must be strong enough to keep the demodulated bit error rateacceptably small (for example, less than about 10⁻⁵), which can beguaranteed only if the received signal power is above approximately −148dBm. Accordingly, it is difficult to establish sub-microsecond timetransfer using very weak GPS/GNSS signals suffering severe attenuation,such as those received indoors or in urban canyons.

BRIEF DESCRIPTION OF THE INVENTION

The embodiments of the present invention provide a method ofsub-microsecond time transfer in a GPS/GNSS receiver using a weakGPS/GNSS signal from a satellite. A digitized complex baseband signal ofthe GPS/GNSS signal carries a PN code (referred to as “received PNcode”) and a navigation message. The GPS/GNSS receiver generates a PNcode for the satellite (referred to as “generated PN code”). Thegenerated PN code is time-aligned with the received PN code. A timingsignal indicating code epochs of the generated PN code is generated,where each code epoch marks a beginning of each period of the generatedPN code (referred to as a “code period”). The baseband signal isreceived, and the baseband signal and the generated PN code arecross-correlated for each code period in accordance with the timingsignal, so as to output a complex correlation value at each code epoch,where a sequence of the complex correlation values forms a data streamrepresenting the navigation message. Bit boundaries of the data streamare located, and numbered bit sync pulses are generated at the bitboundaries. The bit location of a target segment is detected in thenavigation message, where the target segment has a known sequence of kbits at a known bit location in each subframe of the navigation message.The location of the target segment is detected by searching through aplurality of subframes and accumulating search results for the pluralityof subframes. Transmission time of the target segment is determined,with a certain time ambiguity, based on the detected bit location.

Accurate (sub-microsecond) local time at the GPS/GNSS receiver may bedetermined by solving the certain time ambiguity in the transmissiontime, using approximate time obtained from an external source, and bycorrecting a propagation delay.

The digitized complex baseband signal may be Doppler-compensated beforethe cross-correlation, using a frequency-locked loop (FLL). Thegenerated PN code may be aligned with the received PN code bycorrelating the received PN code and the generated PN code, using adelay-locked loop (DLL).

In one aspect of the invention, the detection of the target segmentincludes (a) accumulating the complex correlation values for one bitperiod to obtain a corresponding complex bit value, (b) placing thecomplex bit value in a k-stage complex shift register at a correspondingbit sync pulse, the shift register holding a sequence of most recent kcomplex bit values, (c) obtaining a weighted sum of the sequence of themost recent k complex bit values, each complex bit value being weightedby a respective one of a sequence of weights, the sequence of theweights corresponding to the known bit sequence of the target segment,and obtaining the magnitude of the weighted sum, (d) storing themagnitude of the weighted sum in an accumulator which is associated withthe corresponding bit sync pulse at which the latest complex bit valueis placed into the shift register, (e) iterating steps (a) through (d)until the magnitudes of the weighted sums are placed in a predeterminednumber of accumulators, (f) repeating step (e) for a plurality of timessuch that each of the predetermined number of accumulators accumulatesthe weighted sums, (g) determining one of the accumulators whichcontains a largest accumulated value of the magnitudes and a specificbit sync pulse associated therewith, and (h) identifying a bit locationof the target segment within a subframe based on the specific bit syncpulse. The bit location may be the location of the first bit of thetarget segment.

The step (c) of obtaining the magnitude of the weighted sum may include(c1) performing k-point fast Fourier transform (FTT) on the weightedcomplex bit values to generate k FTT outputs, (c2) calculating magnitudeof each of the k FTT outputs, and (c3) obtaining a maximum value of themagnitude as the magnitude of the weighted sum.

The target segment may be the preamble of the subframe in the navigationmessage, where number k is 8. Alternatively, the target segment may bethe Time of Week (TOW) in the navigation message, where number k is 17.

The certain time ambiguity may be a 6 (±3) second ambiguity. Thepredetermined number of accumulators may be a bank of accumulatorsincluding 300 accumulators.

Other embodiments of the invention also provide a method ofsub-microsecond time transfer in a GPS/GNSS receiver using a weakGPS/GNSS signal from a satellite, in which the target segment isdetected by searching for the target segment through the navigationmessage and comparing a search result with a predetermined thresholdvalue. Transmission time of the target segment is determined, with acertain time ambiguity, based on the bit location.

The detection of the target segment may include (a) accumulating thecomplex correlation values for one bit period to obtain a correspondingcomplex bit value, (b) placing the bit value in a k-stage complex shiftregister at a corresponding bit sync pulse, the shift register holding asequence of most recent k complex bit values, (c) obtaining a weightedsum of the sequence of the most recent k complex bit values, eachcomplex bit value being weighted by a respective one of a sequence ofweights, the sequence of the weights corresponding to the known bitsequence of the target segment, and obtaining the magnitude of theweighted sum, (d) determining if the magnitude of the weighted sumexceeds a predetermined threshold value, (e) repeating steps (a) through(d) if the magnitude of the weighted sum does not exceed thepredetermined threshold value, and (f) identifying the corresponding bitsync pulse at which the latest bit value is placed into the shiftregister as a bit location of the target segment, if the magnitude ofthe weighted sum exceeds the predetermined threshold value.

The step (c) of obtaining the magnitude of the weighted sum may include(c1) performing k-point fast Fourier transform (FTT) on the weightedcomplex bit values to generate k FTT outputs, (c2) calculating amagnitude of each of the k FTT outputs, and (c3) obtaining a maximumvalue of the magnitudes of the k FTT outputs as the magnitude of theweighted sum. A frequency error signal may be generated based on thefrequency associated with the k-point FTT output having the maximummagnitude.

If the target segment occurs once in every frame, the certain timeambiguity is a 30 (±15) second ambiguity. If the target segment occursonce in every navigation message, the certain time ambiguity is a 12.5(±6.25) minute ambiguity.

The target segment may be an ephemeris data segment in the navigationmessage, and number k may be 128 or more.

Embodiments of the present invention also provide specific bitsynchronization by locating bit boundaries using a weak GPS/GNSS signal.There are 20 code epochs per data bit. Locating the bit boundariesincludes (a) placing the complex correlation value in a 20-stage complexshift register at a corresponding code epoch in accordance with thetiming signal, the shift register holding a sequence of most recent 20complex correlation values, (b) summing the 20 complex correlationvalues in the shift register at each epoch, (c) calculating a magnitudevalue of the sum of the 20 complex correlation values, (d) storing themagnitude value in one of 20 accumulators which is associated with thecode epoch at which the latest complex correlation value is placed intothe shift register, (e) iterating steps (a) through (d) until arespective magnitude value is placed in each of the 20 accumulators, (f)repeating step (e) for a plurality of times such that each of the 20accumulators accumulates the magnitude values, (g) determining one ofthe 20 accumulators containing a largest accumulated value of themagnitude values and a specific code epoch associated with theaccumulator containing the largest accumulated value, (h) identifyingthe specific code epoch as a bit boundary of the data stream, and (i)outputting the bit sync pulse occurring every 20 code epochs.

The weak GPS/GNSS signal has a signal level less than about −151 dBm,more generally, less than −148 dBm. The weak GPS/GNSS signal may have asignal level less than −160 to −170 dBm.

Other embodiments of the present invention also provide a GPS/GNSSreceiver having a sub-microsecond time transfer circuit using a weakGPS/GNSS signal from a satellite. The GPS/GNSS signal carries a PN code(referred to as “received PN code”) and a navigation message. TheGPS/GNSS receiver includes a PN code generator for generating a PN codefor the satellite (referred to as “generated PN code”). The PN codegenerator also outputs a timing signal indicating code epochs of thegenerated PN code, each code epoch marking a beginning of each period ofthe generated PN code (referred to as “code period”), a cross-correlatorfor cross-correlating the generated PN code and a digitized complexbaseband GPS/GNSS signal for each code period in accordance with thetiming signal so as to output a complex correlation value at each codeepoch, a sequence of the complex correlation values forming a datastream representing the navigation message, and a bit synchronizer forlocating bit boundaries of the data stream and generating numbered bitsync pulses at the bit boundaries.

In one aspect of the invention, the GPS/GNSS receiver further includes atarget segment locator coupled to the bit synchronizer and thecross-correlator, for detecting a target segment in the navigationmessage, the target segment having a known sequence of k bits at a knownbit location in each subframe of the navigation message. The targetsegment locator searches through a plurality of subframes andaccumulates search results for the plurality of subframes so as toidentify a specific bit sync pulse as a bit location of the targetsegment within a subframe. The GPS/GNSS receiver also includes atransmission time determiner for determining transmission time of thetarget segment with a certain time ambiguity, based on the bit locationof the target segment and known transmission time of a particular bit inthe navigation message.

The GPS/GNSS receiver may further include a local time determiner. Thelocal time determiner includes a time ambiguity resolution section forsolving the certain time ambiguity using approximate time obtained froman external source so as to determine local time with sub-microsecondaccuracy, and a propagation delay correction section.

In one aspect of the invention, the target segment locator include aone-bit accumulator for accumulating the complex correlation values forone bit period to obtain a complex bit value, a k-stage complex shiftregister for placing the complex bit value in accordance with acorresponding bit sync pulse, the shift register holding a sequence ofmost recent k complex bit values, a weighted multiplexer for obtaining aweighted sum of the sequence of the most recent k complex bit values,each complex bit value being weighted by a respective one of a sequenceof weights, the sequence of the weights corresponding to the known bitsequence of the target segment, a bank of accumulators for storing themagnitudes of the weighted sums in accordance with the corresponding bitsync pulses, and a controller for determining a specific accumulator ofthe bank which contains a largest value of the accumulated magnitudes ofthe weighted sums and identifying the bit location of the target segmentbased on a specific bit sync pulse associated with the specificaccumulator.

The weighted multiplexer may include a k-point fast Fourier transformer(FTT) for performing k-point FTT on the weighted complex bit values togenerate k FTT outputs, and a magnitude calculator for calculatingmagnitude of each of the k FTT outputs and obtaining a maximum value ofthe magnitudes of the k FTT outputs as the magnitude of the weightedsum.

The target segment may be the preamble of the subframe in the navigationmessage, and number k is 8. The target segment may be the Time of Week(TOW) in the navigation message, and number k is 17.

The certain time ambiguity may be a 6 (±3) second ambiguity. The bank ofaccumulators may have 300 accumulators.

Other embodiments of the invention provide another target segmentlocator of the GPS/GNSS receiver which is coupled to the bitsynchronizer and the cross-correlator. The target segment locatorlocates a target segment in the navigation message, where the targetsegment has a known sequence of k bits at a known bit location in thenavigation message. The target segment locator searches for the targetsegment through the navigation message so as to detect a bit location ofa target segment by comparing a search result with a predeterminedthreshold value.

The target segment locator may include a one-bit accumulator foraccumulating the complex correlation values for one bit period to obtaina complex bit value, a k-stage complex shift register for placing thecomplex bit value in accordance with a corresponding bit sync pulse, theshift register holding a sequence of most recent k complex bit values, aweighted multiplexer for obtaining a weighted sum of the sequence of themost recent k complex bit values, each complex bit value being weightedby a respective one of a sequence of weights, the sequence of theweights corresponding to the known bit sequence of the target segment, acomparator for determining if the magnitude of the weighted sum exceedsa predetermined threshold value, and a controller for identifying, ifthe magnitude of the weighted sum exceeds a predetermined thresholdvalue, the bit location of the target segment from the bit sync pulse atwhich the latest bit value is placed into the shift register.

The weighted multiplexer may include a k-point fast Fourier transformer(FTT) for performing k-point FTT on the weighted complex bit values togenerate k FTT outputs, and a magnitude calculator for calculatingmagnitude of each of the k FTT outputs and obtaining a maximum value ofthe magnitudes of the k FTT outputs as the magnitude of the weightedsum.

The weighted multiplexer may further include a frequency error signalgenerator for generating a frequency error signal based on a frequencyassociated with the k-point FTT output having the maximum magnitude.

The target segment may occur once in every frame and the certain timeambiguity is a 30 (±15) second ambiguity. The target segment may be anephemeris data segment in the navigation message, and number k may be128 or more. Alternatively, the target segment may occur once in everynavigation message and the certain time ambiguity is a 12.5 (±6.25)minute ambiguity.

In one aspect of the invention, the GPS/GNSS receiver may furtherinclude a frequency-locked loop (FLL) for compensating Doppler errors inthe baseband signal. The GPS/GNSS receiver may also include adelay-locked loop (DLL) for aligning the generated PN code with thereceived PN code by correlating the received PN code and the generatedPN code.

Embodiments of the present invention also provide a specific bitsynchronizer for the GPS/GNSS receiver using a weak GPS/GNSS signal.There are 20 code epochs per data bit. The bit synchronizer includes a20-stage complex shift register for placing the complex correlationvalue at a corresponding code epoch in accordance with the timingsignal, the shift register holding a sequence of most recent 20 complexcorrelation values, an adder for summing the 20 complex correlationvalues in the shift register at each epoch, a magnitude calculator forcalculating a magnitude of the sum of the 20 complex correlation values,a bank of accumulators for storing a magnitude value in accordance withthe code epochs for a certain period of time, a controller determiningone of the accumulators which contains a largest accumulated value ofthe magnitude values and identifying a specific code epoch associatedwith the accumulator containing the largest accumulated value as a bitboundary of the data stream, and a bit sync pulse generator foroutputting bit sync pulse at code epochs corresponding to the bitboundaries.

Other embodiments of the present invention provide a method forestablishing a position of a GPS/GNSS receiver. The method includes (a)observing first and second GPS/GNSS signals from a first pair ofsatellites including first and second satellites, the first GPS/GNSSsignal defining a first point on a first signal structure, and thesecond GPS/GNSS signal defining a second point on a second signalstructure, (b) obtaining a first transmission time (T_(A1)) of the firstpoint in the first GPS/GNSS signal and a second transmission time(T_(B1)) of the second point in the second GPS/GNSS signal, (c) locatinga first position of the first satellite based on the first transmissiontime, and a second position of the second satellite based on the secondtransmission time, (d) computing a difference in the first and secondtransmission times (Δ₁=T_(A1)−T_(B1)), the difference Δ₁ defining a lineof position P_(C1) (Δ₁) on a surface of the earth along which the timedifference is to be observed, using given knowledge of altitude as afunction of latitude and longitude, (e) after a certain time period,repeating steps (a) through (d) for a second pair of satellites atdifferent positions, so as to obtain a second line of position P_(C2)(Δ₂) on the surface of the earth, wherein Δ₂=T_(A2)−T_(B2), T_(A2) andT_(B2) are two transmission times of the second pair of satellites atthe different positions, and (f) obtaining the receiver position at anintersection of the two lines of position P_(C1) (Δ₁) and P_(C2) (Δ₂).The second pair of satellites may include one or both of the satellitesin the first pair of satellites.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and not by wayof limitation, in the figures of the accompanying drawings and in whichlike reference numerals refer to similar elements and in which:

FIG. 1 is a block diagram schematically illustrating a method andcorresponding circuitry for sub-microsecond time transfer in accordancewith one embodiment of the present invention.

FIG. 2A is a schematic timing diagram illustrating the received PN code,the generated PN code, and the corresponding code epochs in accordancewith one embodiment of the present invention.

FIG. 2B is a schematic diagram illustrating the navigation data streamand the corresponding bit sync pulses in accordance with one embodimentof the present invention.

FIG. 2C is a diagram schematically illustrating the navigation datastream and the modulo-300 tags.

FIG. 2D is a diagram schematically illustrating the 8-stage shiftregister and 300 accumulator operations in accordance with oneembodiment of the present invention.

FIG. 2E is a schematic diagram illustrating the accumulated values inthe 300 accumulators.

FIG. 3 is a diagram schematically illustrating an example of the bitsynchronizer and corresponding bit synchronization process, inaccordance with one embodiment of the present invention.

FIG. 4 is a diagram schematically illustrating an example of the frameand subframe structure of the GPS navigation data stream.

FIG. 5 is a diagram schematically illustrating an example of thepreamble locator and a corresponding method for locating the preamble,in accordance with one embodiment of the present invention.

FIG. 6 is a block diagram schematically illustrating a method and thecorresponding circuitry for obtaining sub-microsecond time, inaccordance with another embodiment of the present invention.

FIG. 7 is a diagram schematically illustrating an example of the targetsegment locator 80 and the corresponding method for detecting theoccurrence of a known bit sequence in accordance with one embodiment ofthe present invention.

FIG. 8 is schematic diagram illustrating a method for obtaining thereceiver's position from two GPS signals, in accordance with oneembodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The present invention will now be described in detail with reference toa few preferred embodiments thereof as illustrated in the accompanyingdrawings. In the following description, numerous specific details areset forth in order to provide a thorough understanding of the presentinvention. It will be apparent, however, to one skilled in the art, thatthe present invention may be practiced without some or all of thesespecific details. In other instances, well known process steps and/orstructures have not been described in detail in order to notunnecessarily obscure the present invention.

The present invention provides a new method and apparatus which permitsub-microsecond time transfer using very weak GPS/GNSS signals sufferingsevere attenuation, such as those received indoors or in urban canyons.Depending on several factors, signals as weak as −160 to −170 dBm can beused. Once time transfer has been initiated, it can be maintained usingonly one GPS/GNSS satellite at any given moment, and handovers from onesatellite to another are made possible. Methods will also be describedfor maintaining time accuracy when all satellite signals drop below thetracking threshold.

For concreteness, the embodiments of the present invention are describedusing L₁ C/A (Coarse Acquisition)-coded GPS signals. However, it is wellunderstood by those of ordinary skill in the art that the presentinvention is not limited to the use of specific GPS signal, but can beapplied to other GNSS signals as well. Similarly, a GPS receiver in thefollowing description may be a GNSS receiver.

In the description, it is assumed that the following conditions aresatisfied: (1) the GPS receiver is stationary; (2) The position of theGPS receiver is established by any of various methods; (3) Data such assatellite ephemeris data, satellite clock correction data, almanac data,and other data, such as Time of Week (TOW), in the navigation message isavailable from an external source, such as the internet or an assistingDSL (ADSL) link; (4) Once initiated, the time transfer is continuouslyavailable, but relatively long time intervals are permitted for itsinitiation. Intervals on the order of 10 minutes to tens of hours mightbe acceptable for this purpose. In addition, the GPS receiver and itstime transfer hardware/software should be low-cost. Also, low powerconsumption is not necessary.

FIG. 1 is a block diagram schematically illustrating a method andcorresponding circuitry 100 for sub-microsecond time transfer from asingle satellite in accordance with one embodiment of the presentinvention. If more satellite signals are available, all elements in thefigure are replicated for each satellite tracked. FIG. 1 only shows partof a GPS receiver which is able to obtain very accurate time, i.e.,sub-microsecond time from a very weak GPS signal. It is assumed that theposition of the receiver is known within approximately 100 meters forsub-microsecond accuracy, and that a satellite signal has already beenacquired and code and carrier frequency tracking have been initiated.That is, as shown in FIG. 1, the input signal to the circuitry 100 is adigitized baseband signal 10 which is received by a radio frequency (RF)front end portion (not shown) of the GPS receiver and has been convertedinto the baseband frequency.

The signal acquisition, code tracking, and carrier frequency trackingmay be performed using any conventional processes well known to those ofordinary skill in the art. The signal acquisition, code tracking, andcarrier frequency tracking are not part of the present invention andthus they are not described in this specification in detail. Methods ofinitially establishing the position of the receiver will be discussedlater. It should be noted that the digitized baseband signal 10 is acomplex signal and may be expressed as I+jQ (I: real or cosinecomponent, Q: imaginary or sine component). The signal processing isthus carried out in two channels, although figures do not explicitlyshow two channels for simplicity.

Frequency-Locked Loop

In order to track very weak signals, carrier tracking is accomplished bymeans of a frequency-locked loop (FLL) with very narrow bandwidth (onthe order of 0.01 Hz), which is much narrower than conventional FLL forGPS signals. It should be noted that traditional Costas phase-lockedloops (PLL's) are not suitable for tracking very weak signals (belowabout −151 dBm), because they suffer too much SNR loss due to the signalsquaring that is necessary to eliminate data-bit-caused noncoherence. Itmight be thought that the SNR loss could be regained by using anextremely small PLL loop bandwidth (perhaps as small as 0.01 Hz).However, at such small loop bandwidths, the PLL can have difficultytracking the signal phase instability after frequency conversion tobaseband. The phase instability is caused by the phase instability ofthe typical low-cost receiver TCXO reference oscillator that serves as afrequency reference in the receiver.

Referring to FIG. 1, the complex (I+jQ) digitized baseband GPS signal 10is Doppler compensated by a phase rotator 12 which is controlled by theFLL 14 to drive the signal frequency nominally to zero frequency at thephase rotator output 16. The FLL 14 includes a frequency discriminator18, an FLL loop lowpass filter 20, and a numerically-controlledoscillator (NCO) 22. The input signal 24 to the frequency discriminator18 consists of the complex outputs (I+jQ) of 1-millisecond signalcorrelations (correlator) 28, which is to be described below in moredetail. The frequency discriminator 18 is designed to minimize theeffect of polarity transitions in the received 50 bit/sec navigationdata so that it will pull in even when the timing of the data bitboundaries is unknown.

Because the receiver is stationary, is at a known location (approximatelocation), and ephemeris data is available, a known Doppler ratecorrection 26 is applied to the phase rotator 12 via the NCO 22 as shownin FIG. 1. The Doppler rate correction may be no greater than 1 Hz/sec,and is not sensitive to moderate errors in the known receiver position.This removes essentially all frequency change on the signal due tosatellite motion and enhances the operation of the FLL 14 which detectsand removes the residual Doppler error by controlling the NCO 22 via theFLL loop filter 20.

Simulations show that the FLL can track a signal as weak as −165 dBmwith a maximum frequency tracking error of about ±10 Hz, and a pull-intime constant on the order of several minutes. This high level ofperformance is made possible by the stationarity of the receiver and theDoppler rate correction of the NCO 22, both of which keep the signalfrequency essentially constant. The frequency tracking error is smallenough to permit proper operation of the time extraction to be describedlater.

Code Tracking Delay-Locked Loop (DLL)

FIG. 1 also shows a code tracking delay-lock loop (DLL) 30 whichincludes a code tracking discriminator 32, a DLL lowpass loop filter 34,and a code generator 36 which produces a locally generated replica 38 ofthe received C/A-PN (Pseudorandom Noise) code. The code trackingdiscriminator (correlator) 32 correlates the generated PN code 38 andthe received PN code (in the Doppler compensated baseband signal 16) andoutputs a code phase error signal 33 if the generated PN code 38 and thereceived PN code 16 are not aligned. The DLL loop filter 34 removesnoise in the code phase error signal 33 and applies the signal as a codephase control signal 35 to the code generator 36. The code generator 36outputs the generated PN code 38 thus aligned. That is, the DLL 30controls the code generator 36 to keep the replica PN code 38time-aligned with the received PN code 16 appearing at the output of thephase rotator 12. The code generator 36 also outputs a code epoch signal(timing signal) 39 at each epoch of the locally generated PN code 38,where an epoch occurs every millisecond.

During a normal operation when sub-microsecond time is available, thesensitivity of the DLL tracking loop 30 can be significantly enhancedbecause the receiver's known location, the ephemeris data, and accurateknowledge of time permit the Doppler trajectory of the satellite to bepredicted very accurately. It is expected that the DLL 30 will be ableto track signals as low as −170 dBm using a very narrow loop bandwidth(about 0.01 Hz or less).

Millisecond Correlations

As shown in FIG. 1, the 1-msec correlations (correlator) 28cross-correlates the signal 16 (received PN code) at the phase rotatoroutput with the PN code 38 coming from the code generator 36 to producea continuing sequence of complex-valued (I+jQ) 1-millisecond correlationoutputs (complex correlation values) 24. The output of each correlationand the beginning of the next correlation occur at each epoch of thelocally generated code, where an epoch occurs every millisecond. Thatis, controlled by the code epoch signal (timing signal) 39, the 1-mseccorrelator 28 cross-correlates the baseband signal 16 and the generatedPN code 38 for each code period (i.e., 1 millisecond or 1023 chips) andoutputs the complex correlation value at each code epoch. The sequenceof the complex correlation values (I+jQ) forms a data streamrepresenting the navigation message.

Because the DLL 30 keeps the locally generated PN code 38 aligned withthe received PN code 16, the output of each correlation is near the peakof the correlation function, thereby giving enough processing gain toenable the ensuing FLL, bit synchronization, and time transferoperations. FIG. 2A schematically illustrates the received PN code, thealigned generated PN code, and the corresponding code epochs.

Bit Synchronization

As is well known to those of ordinary skill in the art, the 50-bps (bitper second) data stream of the GPS signal conveys the navigationmessage. The 50-bps data bit boundaries always occur at an epoch of thePN code. The PN code epochs mark the beginning of each period (1millisecond, 1023 chips) of the PN code, and there are precisely 20 codeepochs per data bit (20 milliseconds, 20,460 chips). In order to operateat extremely low signal levels where phase cannot be reliably tracked, along-term partially coherent method of bit synchronization is performedat a bit synchronizer 40. The bit synchronizer 40 locates bit boundariesin the GPS data stream and generates numbered (tagged) bit sync pulsesat the bit boundaries in accordance with one embodiment of the presentinvention.

FIG. 3 schematically illustrates an example of the bit synchronizer 40and corresponding bit synchronization process, in accordance with oneembodiment of the present invention. As shown in FIG. 3, the bitsynchronizer 40 includes a 20-stage complex shift register 50, an adder52, and 20 accumulators 56. The complex (I+jQ) correlation values 24output from the 1-millisecond correlator 28 (shown in FIG. 1) enter the20-stage shift register 50 at each occurrence of an epoch of the locallygenerated PN code 38, so that the shift rate is nominally 1000shifts/second. The epochs provided by the code epoch signal 39 alsocontinuously drive a modulo-20 counter (not shown), which tags eachepoch with a modulo-20 number (0-19), as shown in FIG. 2A. Referringback to FIG. 3, for each shift the complex correlation values in the 20stages of the shift register 50 are summed by the adder 52, and themagnitude of the sum is taken by the magnitude calculator 54. Themagnitude value 55 of the sum is placed into one of the 20 accumulators56, where the specific accumulator 57 n (0≦n≦19) has the same index n asthe modulo-20 tag of the latest PN code epoch.

The magnitude values 55 summed into the accumulators 56 will tend to belargest when a full bit of the signal data resides in the shift register50, since the bit values (correlation values) remain the nearly the same(except for noise) within the same bit. This condition, which isrepeated every 20 milliseconds, always occurs at the same modulo-20 tagof the PN code epochs. At other epochs, data bit polarity transitionswill frequently appear within the shift register 50, tending to reducethe magnitude values 55. Thus, after a sufficiently long time (repeatingthe accumulating process), the index n of the accumulator containing thelargest accumulated value 57 n will be the modulo-20 tag n of the codeepochs where the data bit transitions occur (i.e. the bit boundary). Thebit sync pulse generator 58 of the bit synchronizer 40 can now produce abit sync pulse 44 each time these specific code epochs occur. Data bitsend and start at the bit sync pulse 44. FIG. 2B schematicallyillustrates an example of the navigation data stream and correspondingbit sync pulses. The bit synchronizer 40 may also include a controller(not shown) which determines one of the accumulators 56 containing thelargest accumulated value and identifies the specific code epochassociated with that accumulator as the bit boundary of the data stream.The controller may be part of the bit sync pulse generator 58.

Simulations show that this method of bit sync is very reliable at −160dBm when allowed to run for 1 minute, at −168 dBm when run for 10minutes, and at −170 dBm when run for 30 minutes, even when the FLLfrequency tracking error is as great as ±10 Hz.

Obtaining Signal Transmission Time

The key to obtaining sub-microsecond time is the ability to determinethe Space Vehicle (SV) time at which any given point on the receivedsignal was transmitted. The transmission times of certain parts of thesignal are known a-priori, but generally with some time ambiguity whichcan be resolved by various methods to be described. If the occurrence ofthose parts of the signal can be detected, it is then possible todetermine the SV time when any part of the signal was transmitted. Clockcorrection data from an external source can then convert the SV time tovery accurate GPS time. As is well known to those of ordinary skill inthe art, time defined by the clocks in the satellite is commonlyreferred to as SV time, and the time after corrections have been appliedis referred to as GPS time. Thus, even though individual satellites maynot have perfectly synchronized SV times, they share a common GPS time.

Example 1: Detecting Occurrence of the Subframe Preamble

FIG. 4 schematically illustrates the frame and subframe structure of theGPS navigation data stream. A complete navigation data message consistsof 25 frames, each containing 1500 bits transmitted at a 50 bit/secrate. Each frame is subdivided into five 300-bit subframes (#1 through#5 in FIG. 4), each subframe consisting of 10 words of 30-bits each,with the most significant bit (MSB) transmitted first. Thus, at the 50bits/sec rate, it takes 6 seconds to transmit a subframe, and 30 secondsto complete one frame. Transmission of the complete 25-frame navigationmessage requires 750 seconds, or 12.5 minutes. Except for occasionalupdating, subframes #1, #2, and #3 are mostly constant (i.e., repeat)with each frame at the 30 second frame repetition rate. On the otherhand, subframes #4 and #5 are each subcommutated 25 times. The 25versions of subframes #4 and #5 are referred to as pages 1-25. Hence,except for occasional updating, each of these pages repeats every 750seconds, or 12.5 minutes.

The first 8 bits of each subframe is a fixed 8-bit sequence (1 0 0 0 1 01 1) called the preamble, which can be used to locate the beginning ofeach subframe. The SV transmission time of the first bit in eachpreamble is a known time, except for a 6-second ambiguity. Thus, thetransmission time of the first bit modulo 6 seconds can be establishedif the occurrence of the preamble can be detected within the navigationdata bit stream.

The occurrence of the preamble is detected by a preamble locator 60(shown in FIG. 1). FIG. 5 schematically illustrates an example of thepreamble locator 60 and the corresponding method for locating thepreamble, in accordance with one embodiment of the present invention.The preamble locator includes a 20 millisecond accumulator 61, an8-stage complex shift register 62, a weight multiplier 64, an optional8-point fast Fourier transformer (FFT) 66, a magnitude calculator 68, amaximum value selector 70, and a bank of three hundred (300)accumulators 72. The weight multiplier 64, the optional 8-point fastFourier transformer (FFT) 66, the magnitude calculator 68, and themaximum value selector 70 may together be referred to as a weightedmultiplexer. As shown in FIG. 5, the complex correlation values 42output from the millisecond correlations 28 of FIG. 1 are accumulated inthe 20 millisecond accumulator 61.

Assuming that the bit synchronization has been established, as discussedabove, each 20 millisecond accumulation represents the sum of thecomplex values within the same data bit, which corresponds to thecomplex value representing bit 1 or 0. The 20-millisecond accumulations(i.e., corresponding to complex bit values) are passed through the8-stage shift register 62 at the timing of the bit sync pulse 44. Thatis, each occurrence of the bit sync pulse 44 dumps the 20-millisecondaccumulation into the shift register 62 and simultaneously resets andrestarts the accumulation process at the 20 millisecond accumulator 61.Thus, the contents of the shift register 62 are shifted every 20milliseconds, i.e., bit by bit. The bit sync pulses 44 also drive amodulo-300 counter (not shown), which tags each bit sync pulse with amodulo-300 number (0-299) as shown in FIG. 2C. Each complex bit value isalso identified by the corresponding modulo-300 tag of the bit syncpulse 44 at which the complex bit value is shifted into the shiftregister 62 from the 20-millisecond accumulator 61. The 8-stage shiftregister 62 holds complex bit values corresponding to 8 data bits,serving as an 8-bit window for searching the known bit sequence of thepreamble. As shown in FIG. 5, for example, at the time when the bit syncpulse 44 has a modulo-300 tag m, the shift register 62 holds complex bitvalues corresponding to the modulo-300 tags from m−7 to m.

The output taps (complex values) of the 8-stage shift register 62 areweighted by the known preamble bit sequence (with a weight of −1replacing the binary 0) at the weight multiplier 64, as shown in FIG. 5.It should be noted that, when the data bit value changes, a complexvector representing the bit value flips in the complex plane. Thus, whenthe preamble fully occupies the shift register 62, the weighting by theknown bit sequence of 1 and −1 (except noise) make all of the vectorsfor 8 bits identical (except for noise). If there is no Doppler error,the weighted complex values (i.e., aligned vectors) may be simply summed(operation of the 8-point FTT may be inhibited), and the magnitude ofthe sum tends to be largest when the 8-stage shift register 62 containsthe preamble. However, the FLL 14 (shown in FIG. 1) may not perfectlycompensate the Doppler effects, and Doppler compensation at the phaserotator 12 may not be perfect due to noise, so the complex vector maystill be slowly rotating. Accordingly, as shown in FIG. 5, immediatelyafter each shift, an 8-point FFT (66) is performed on the weightedoutputs, the magnitudes of the 8-point FFT outputs are computed (68),and the maximum value of the magnitudes of the 8-point FFT outputs isselected (70). The maximum value is placed into one of the 300accumulators 72. The specific accumulator 72 m has the same index m asthe modulo-300 tag of the latest bit sync pulse (0≦m≦299), as shown inFIG. 5.

FIG. 2D schematically illustrates an example of the operation of placingthe magnitude of the weighted sum (the maximum value thereof after the8-point FFT) of eight complex bit values to the accumulators. In FIG.2D, A, B, C . . . represent complex bit values (i.e., the 20-millisecondaccumulation corresponding to bit 1 or 0) held in the shift register. Inthis example, as shown in FIGS. 2C and 2D, the shift register containsthe full preamble at the sync pulse with the moulo-300 tag m, where thecomplex bit value B having the moulo-300 tag m−7 corresponds to thefirst (start) bit of the preamble. This process may be repeated a numberof times in order to average out the noise.

When the preamble fully occupies the shift register 62 (which happensevery 300 bits of the navigation message), the maximum magnitude of the8-point FFT outputs tends to be largest, and over time the correspondingaccumulator 72 m (FIG. 5) will build up a value larger than that of theother accumulators. Thus, after a sufficiently long time, the index m ofthe accumulator 72 containing the largest accumulated value will be themodulo-300 tag m of the bit sync pulses 44 at which times the preamblebecomes fully within the shift register 62, as shown in FIG. 2E.

The preamble locator 60 also includes a controller 74 to determine aspecific accumulator 72 m of the bank of the 300 accumulators whichcontains the largest value, and to identify the bit sync pulse havingthe index m−7 as the location of the first bit of the preamble. Thecontroller 74 may include a transmission time determiner (not shown) fordetermining transmission time of the target segment with a certain timeambiguity, based on the bit location of the target segment and knowntransmission time of a particular bit (i.e., the first bit of thesubframe) in the navigation message. In this way the transmission timeof the first bit of the preamble, which is also the first bit in thesubframe, can be determined with a 6-second ambiguity.

If the FLL 14 could maintain a zero frequency error, as mentioned above,the 8-point FFT 66 would not be necessary, because the maximum magnitudeof the 8-point FFT outputs would tend to occur in the zero-frequency FFToutput bin (the output of this frequency bin is just the sum of theweighted outputs of the shift register taps). However, because noisecauses a nonzero frequency error in the FLL tracking by the FLL 14,there can be a signal loss due to reduced coherence in summing theweighted outputs. The FFT regains the coherence by neutralizing thefrequency error in forming one of its outputs.

Since the preamble contains only 8 bits, there is the possibility thatthe same sequence of bits will appear in other parts of the navigationmessage. However, it is very unlikely this will happen at the same spotin every subframe. Since the preamble appears in every subframe, itsoccurrence will be dominant.

Computer simulations show that the preamble can be reliably detected at−160 dBm by observing only 5 frames (150 seconds or 2.5 minutes ofdata), and at −170 dBm by observing 25 frames (750 seconds or 12.5minutes of data).

The requirement of 300 accumulators 72 to locate the preamble can bemitigated in various ways. One method is to search smaller portions ofthe 300 bit subframe period, one portion at a time. For example, if eachportion contains 50 bits, the same 50 bit positions of each subframecould be searched using 50 accumulators, and the maximum accumulatorvalue retained. This process would be repeated 6 times to cover the full300-bit subframe period, after which the maximum of the 6 maxima wouldlocate the preamble. Of course, this would increase the search time by afactor of 6.

Another method could be used if approximate time is available from anoutside source. For example, if time is known to within ±0.5 seconds,only about 50 bits of data need to be searched every 6 seconds, and only50 accumulators would be required. Both methods could be used to furtherreduce the number of accumulators. For example, the 50 bits of datacould be divided into 5 portions of 10 bits each, reducing the number ofaccumulators to 10.

Example 2: Detecting the Occurrence of a Larger Segments of Bits

FIG. 6 schematically illustrates an alternative way of obtaining signaltransmission time, in accordance with another embodiment of the presentinvention, which detects the occurrence of larger segments of data bitsin the navigation message. In FIG. 6, the like elements bear the likereference numerals as those in FIG. 1. As shown in FIG. 6, the circuitry200 includes a target segment locator 80 in place of the preamblelocator 60 in FIG. 1. Since it is assumed that satellite ephemeris data,satellite clock correction data, almanac data, and perhaps other data,such as Time of Week (TOW), in the navigation message is available froman external source, this information can be used to determine the bitsequences that are used to transmit it. Since the position of theseknown bit sequences relative to the start of a frame is known, theoccurrence of the first bit at the start of a frame can also bedetermined from the detected known bit sequence. Since the transmissiontime of the start-of-frame bit is known with an ambiguity of 30 seconds(the length of a frame), the ambiguity can easily be removed byindependent knowledge of time to within ±15 seconds.

FIG. 7 schematically illustrates an example of the target segmentlocator 80 and the corresponding method for detecting the occurrence ofknown bit sequence in accordance with one embodiment of the presentinvention. For concreteness, it is assumed that a 128-bit sequence withknown position relative to the start of the frame has been determinedfrom externally supplied navigation data. A certain number of data bitshaving a known sequence at a known position within a frame is referredto as a target segment. Thus, the preamble in the previous embodimentmay also be a target segment. In this example, the target segment is a128-bit sequence of the ephemeris data. The number of bits assumed isnot limiting but illustrative, and a smaller or larger number can beused. However, in this example, a relatively large number has beenchosen to illustrate the large processing gain that can be achieved. Asshown in FIG. 7, the target segment locator 80 includes a 20 millisecondaccumulator 81, a 128-stage complex shift register 82, a weightmultiplier 84, an optional 128-point fast Fourier transformer (FFT) 86,and a magnitude calculator 88. The weight multiplier 84, the optional128-point fast Fourier transformer (FFT) 86, and the magnitudecalculator 88 may together be referred to as a weighted multiplexer. Thetarget segment locater 80 also includes a controller 90 having a maximumvalue comparer.

The complex outputs 24 of the 1-millisecond correlations 28 (shown inFIG. 6) are accumulated in the 20 millisecond accumulator 81, and theoutputs of the 20 millisecond accumulator 81, which occur every 20milliseconds, are passed through the 128-stage shift register 82. Eachoccurrence of the bit sync pulse 44 dumps the 20-millisecondaccumulation into the shift register 82, and simultaneously resets andrestarts the accumulation process in the 20 millisecond accumulator 81,in a similar manner as the previous example shown in FIG. 5. Thus, thecontents of the shift register 82 are shifted every 20 milliseconds. Thebit sync pulses 44 also drive a modulo-1500 counter (not shown), whichtags each bit sync pulse 44 with a modulo-1500 number (0-1499).

The output taps of the 128-stage shift register 82 are weighted by theknown bit sequence from the externally supplied navigation data at theweight multiplexer 84. Similarly to the previous example, data bit 0 isreplaced with −1 in the weighting Immediately after each shift, a128-point FFT 86 is performed on the weighted outputs, and the magnitudeof each of the 128 FFT outputs is computed (88). The maximum magnitudeof the 128 FFT outputs is compared with a predetermined threshold T atthe maximum value comparer in the controller 90. Since the targetsegment has a sufficiently large number of bits in a particular sequence(which tends to be random or non-repetitive), the wholly containedsegment in the 128-stage shift register 82 would produce a significantlygreater value than any other cases. Thus, the threshold T may be set,for example, based on the data length of the target segment and expectednoise level.

If the threshold T is exceeded, it is determined that the known bitsequence of the target segment (ephemeris data in this example) iswholly contained in the shift register 82, and the modulo-1500 tag ofthe latest bit sync pulse 44 is recorded. Since the location of theknown bit sequence relative to the first bit of the frame is known, themodulo-1500 tag of the bit sync pulses at the beginning of the first bitof the frames is also now known, and the transmission times of thesefirst bits are known with a 30 second ambiguity. In this way, thecontroller 90 identifies, if the maximum value (the weighted sum)exceeds the predetermined threshold T, the bit location of the targetsegment from the recorded bit sync pulse.

The specific target sequence only occurs once in every frame (i.e., inevery 30 seconds), while the preamble in the previous example occursonce in every subframe (i.e., in every 6 seconds). Thus, if theapproximate time is known within 30 seconds (±15 seconds) from anexternal source, the accurate transmission time can be determined Inaddition, as soon as the maximum magnitude exceeds the threshold T, thetarget segment location can be declared, without searching through theremaining frame.

This method is very reliable because of the large processing gain (about21 dB) inherent in the cross-correlation of the known bit sequence withthe signal, since a relatively large number of bits (compared to 8 bitsof the preamble) are summed Furthermore, the probability is very smallthat the same bit sequence will appear in other places within the frame.Additionally, no accumulator banks are required, substantially reducingcost of the receiver.

The 128-point FFT 86 serves the same purpose as the 8-point FFT 66 usedto locate the preamble, as described previously. However, because of thelarge processing gain, the 128-point FFT 86 can serve another valuablepurpose. When the threshold T is exceeded, the frequency bin with thelargest output magnitude provides a very accurate indication of the FLLfrequency error, because the frequency resolution of the 128-point FFTis 1/(128×0.02)=0.39 Hz. This error can be sent as a high accuracyfrequency error signal 98 to the NCO 22 at 30-second intervals, as shownin FIG. 6, to substantially enhance the operation of the FLL 14, infact, enabling tracking of the signal at −170 dBm. Thus, the controller90 may also include a frequency error signal generator to output thehigh accuracy frequency error signal 98.

Satellite clock correction data present in subframe #1, which alsooccurs once in every frame, can be used in the same way as ephemerisdata, again resulting in determination of transmission time with a 30second ambiguity.

If the known bit sequence is from the almanac, the time ambiguity can beincreased to 12.5 minutes, because the known bit sequence will occuronly in one of the 25 frames constituting the complete navigationmessage. However, it may take up to 12.5 minutes to detect the sequence.

Another bit sequence that can be known is the Time of Week (TOW) in thenavigation message. The TOW consists of the first 17 bits of the second30-bit word in each subframe. If approximate time is known to ±3seconds, the 17-bit pattern can be determined and its occurrencedetected. The result is unambiguous transmission time at any point onthe received signal.

Time Ambiguity Resolution and Correction for Signal Propagation Time

As shown in FIGS. 1 and 6, the final step in obtaining sub-microsecondlocal time is resolution of the remaining time ambiguity and correctingfor signal propagation delay which are performed by a propagation delaycorrection and a time ambiguity resolution section of the local timedetermination circuit 102.

1. Resolution of Time Ambiguity

The time ambiguity is resolved using knowledge of approximate time atthe receiver, which can come from various sources.

For Example 1 of obtaining transmission time (by detecting thepreamble), approximate time must be within ±3 seconds, and for Example 2(by detecting a target segment from the externally supplied navigationdata), it must be within ±15 seconds, which is easier to accomplish,when the target segment occurs once in every frame. If Example 2 uses aknown bit sequence from the almanac, approximate time only needs to beknown within ±6.25 minutes. These accuracies, especially the latter, arewithin the range of a low-cost clock, such as that found inquartz-controlled wristwatches that have been initially set withsufficient accuracy and maintained for periods of days or weeks (theseclocks can also be calibrated for rate of error to permit the requiredaccuracy over much longer periods). Approximate time to ±3 seconds mightalso be obtained via ADSL, or possibly over cellular telephone networks.The ±6.25 minute accuracy and perhaps the ±15 second accuracy can beobtained via the Internet. Another source of approximate time is thereception of atomically regulated low-frequency radio signals fromstations such as WWVB in the United States or its counterparts in Japanand Europe. The time accuracy available from these sources is generallywithin the 0.01-0.1 second range if no attempt is made to correct forpropagation delay.

Approximate time can also be established by tracking only two satellitesif the receiver position is known, ephemeris data is available, andaltitude as a function of latitude and longitude is known (for example,from a map database). The information of the receiver position(latitude, longitude, and altitude) and the ephemeris data may beautomatically or manually obtained from the Internet, by connecting thereceiver to the Internet via wireless or wired network, or othercommunication links The position information may also be obtained usingthe GPS receiver itself. For example, if the receiver is in a house orbuilding (“indoor”) and thus is receiving a very weak GPS signal, thereceiver may be moved outside or to a nearby location with good signalreception (within an acceptable position accuracy, for example, 100meters) so as to record the current receiver position. As long as thereceiver remains within a range of acceptable position accuracy, thetransfer time detection and the time ambiguity resolution can beperformed in the same manner.

The method of the time ambiguity resolution is similar to the method ofobtaining time from 5 or more satellites when the receiver position isnot initially known and the signals are too weak to recover navigationdata, as described in Magellan Systems Japan, Inc. Patent ApplicationSerial Number 11/103,499, filed in the United States Patent andTrademark Office by Applicant on Apr. 12, 2005, now U.S. Pat. No.7,362,265, issued Apr. 22, 2008. The method is based on the fact thatwith a known receiver position, the modulo-20 millisecond difference intransmission times of the two simultaneously observed signals is a knownfunction of approximate time. Approximate time can be calculated byevaluating the inverse of this known function at the measured modulo-20millisecond difference in transmission times. Expected accuracies willgenerally be within several seconds.

2. Correction for Propagation Delay

Although the above Examples 1 or 2, followed by time ambiguityresolution, can determine the unambiguous GPS transmission time of anypoint on the observed received signal with sub-microsecond accuracy,time at the receiver is the sum of the transmission time and thepropagation time from the satellite to the receiver. The propagationtime is easily calculated, since the position of the satellite at thetransmission time is accurately known, and also the receiver position isassumed to be known. This enables the calculation of thesatellite-to-receiver distance, which when divided by the speed oflight, gives the signal propagation time. The accuracy in establishinglocal time depends on how accurately the position of the receiver isknown (see next section).

Methods of Initially Establishing the Initial Receiver Position

The accuracy of the time transfer will depend on how accurately theposition of the receiver is known. For sub-microsecond accuracy, theposition uncertainty is preferably less than approximately 100 meters.For ±10 microsecond accuracy, a position uncertainty less thanapproximately 1000 meters is acceptable.

The receiver's fixed position can be obtained in various ways. Onemethod is to use assisted indoor positioning technology such as thatrecently developed by Magellan Systems Japan, Inc., with a typicalpositioning accuracy of less than 100 meters, as mentioned above.Another method is to locate the receiver coordinates on a map providedby internet services such as Google Earth. If this method is used, thereceiver coordinates must somehow be entered into the GPS receiver. Yetanother method is the use of positioning technology for cellulartelephones if its positioning accuracy is sufficient (in many cases itwill not be).

A fourth method of establishing the receiver's position with as few as 2satellites can be used if enough time is available (perhaps an hour ormore) and altitude as a function of latitude and longitude is available,in accordance with one embodiment of the present invention. For example,due to buildings or other structures, only 2 satellite signals may beavailable. FIG. 8 schematically illustrates the method for obtaining thereceiver's position from two GPS signals, in accordance with oneembodiment of the present invention. The steps of this method, assumingthat 2 satellites are being tracked, are as follows:

-   -   1. Observing the two GPS signals from the two satellites 122 (A)        and 124 (B) simultaneously, which defines a point on each signal        structure, and obtain the two GPS transmission times T_(A1) and        T_(B1) of the respective points using a technique such as        Example 1 or Example 2 described above, including resolution of        time ambiguity.    -   2. Using the two GPS transmission times T_(A1) and T_(B1) and        the ephemeris data to accurately locate the positions P_(A1) and        P_(B1) of the two satellites.    -   3. Computing the difference in the two transmission times        (Δ₁=T_(A1)−T_(B1)). This difference Δ₁, together with the        knowledge of altitude as a function of latitude and longitude,        defines a line of position (LOP) P_(Δ1) (Δ₁) on the surface of        the earth along which this time difference would be observed.    -   4. After a sufficiently long time (perhaps an hour or more),        repeating steps 1 through 3 (the same two satellites at        different positions P_(A2) and P_(B2) or a different pair of        satellites may be used). The result is a second LOP P_(C2) (Δ₂)        on the surface of the earth, where Δ₂=T_(A2)−T_(B2), T_(A2) and        T_(B2) are the two GPS transmission times of the two satellites        in a different geometry after a certain time period, or of a        different pair of two satellites that gives a different        geometry.    -   5. Obtaining the receiver position at the intersection P_(C) of        the two LOP's.

The accuracy of this method depends on the satellite-receiver geometry.Dilution of Precision (DOP) calculations can be used to assess thepositioning accuracy. Time for signal acquisition can be reduced byusing low frequency atomically-controlled radio signals such as the 60kHz signal from WWVB in Fort Collins, Colo., to accurately calibrate theTCXO reference oscillator in the receiver.

Total Loss of GPS Signals

If all GPS signals are lost, maintenance of local time accuracy willdepend on the stability of the receiver's reference oscillator (usuallya low-cost TCXO), unless an external source of timing information isavailable. During normal operation, the long term TCXO frequency driftcan be very precisely calibrated, even using just one satellite, becausethe receiver knows time very accurately. The length of time the TCXO can“free-wheel” until a 1-microsecond time error develops is equal to thereciprocal of the calibration accuracy in parts per million (ppm). Forexample, if the TCXO has been calibrated to within 0.01 ppm, it can runautonomously for about 1/0.01=100 seconds before developing a±1-microsecond error.

Another strategy for handling total signal losses is to uselow-frequency atomically-controlled radio signals if they are available.For example, the 60 kHz signal from WWVB in Fort Collins, Colorado istransmitted with a stability of 1 part in 10¹². A clock phase-locked tothis received signal probably can maintain sub-microsecond timeindefinitely once the clock has been set during normal operation whenGPS signals are available. However, the stability of the WWVB signalpropagation paths should be investigated.

The extreme sensitivity of the tracking methods described above,especially when the FLL error signal is derived as in Example 2, mightwell make total GPS signal dropouts quite rare in most applications.

Satellite-to-Satellite Handovers

Because the GPS satellites rise and set, the receiver must be able toseamlessly transfer its tracking from one satellite to another. Thisshould not be a problem, because in normal operation even with only onesatellite, the TXCO frequency is calibrated quite accurately, time isknown at the sub-microsecond level, and the distance to any satellitecan be accurately computed. This should permit acquiring a new satellitewith essentially no searching in either frequency or code phase.

While this invention has been described in terms of several preferredembodiments, there are alterations, permutations, modifications, andvarious substitute equivalents, which fall within the scope of thisinvention. It should also be noted that there are many alternative waysof implementing the methods and apparatuses of the present invention. Itis therefore intended that the following appended claims be interpretedas including all such alterations, permutations, and various substituteequivalents as fall within the true spirit and scope of the presentinvention.

what is claimed is:
 1. A method of sub-microsecond time transfer in aGPS/GNSS receiver using a weak GPS/GNSS signal from a satellite, themethod comprising: receiving a digitized complex baseband signal of theGPS/GNSS signal, the baseband signal carrying a received PN code havinga code period and a navigation message having data bits, each data bitcorresponding to twenty code periods; generating a generated PN code forthe satellite and keeping the generated PN code time-aligned with thereceived PN code; generating a timing signal indicating code epochs ofthe generated PN code, each code epoch marking a beginning of eachperiod and being tagged with a modulo-20 number; cross-correlating thebaseband signal and the generated PN code for each code periodcontrolled by the timing signal, so as to output a complex correlationvalue at each code epoch, a sequence of the complex correlation valuesforming a navigation data stream representing the navigation message;locating data bit boundaries of the navigation data stream andgenerating bit sync pulses at the bit boundaries, locating data bitboundaries including: (a1) receiving the navigation data stream andplacing the complex correlation values in a 20-stage complex shiftregister at a timing of the corresponding code epochs, the shiftregister holding a sequence of most recent 20 complex correlationvalues; (b1) summing the 20 complex correlation values held in the shiftregister at each code epoch and obtaining a magnitude value of the sumof the 20 complex correlation values; (c1) storing the magnitude valuein one of 20 accumulators which is associated with the modulo-20 numbercorresponding to that of the code epoch at which the latest complexcorrelation value of the summed 20 complex correlation values was placedinto the shift register; (d1) iterating steps (a1) through (c1) untileach of the 20 accumulators has a respective magnitude value placedtherein; (e1) repeating step (d1) for a plurality of times such thateach of the 20 accumulators accumulates a certain number of themagnitude values; (f1) determining which one of the 20 accumulatorscontains a largest accumulated value of the magnitude values and thespecific modulo-20 number associated therewith; (g1) identifying thecode epoch having the specific modulo 20 number as a data bit boundaryof the navigation data stream; and (h1) outputting the bit sync pulsesat a timing of the code epochs tagged with the specific modulo 20number, each bit sync pulse being tagged with a modulo-M number;detecting a target segment in the navigation message and determining adata bit location of the target segment in the navigation message;determining transmission time of the target segment from the satellitebased on the determined data bit location, wherein the transmission timehas a certain time ambiguity; solving the certain time ambiguity usingapproximate time obtained from a source external to the GPS/GNSSreceiver; and correcting the transmission time for a propagation delayso as to obtain sub-microsecond time transfer.
 2. The method accordingto claim 1, wherein the detecting the target segment and determining thedata bit location of the target segment in the navigation messageincludes: (a2) receiving the navigation data stream and summing thecomplex correlation values for one data bit period in accordance withthe bit sync pulse to obtain a complex bit value for each data bit; (b2)placing the complex bit value in a k-stage complex shift register at atiming of the corresponding bit sync pulse, the k-stage shift registerholding a sequence of most recent k complex bit values; (c2) obtaining aweighted sum of the sequence of the most recent k complex bit values,wherein each of the k complex bit values is weighted by correspondingone of a sequence of k weights, the sequence of k weights correspondingto a known k bit sequence of the target segment; and (d2) determiningthe data bit location of the target segment based on the weighted sumand the corresponding bit sync pulse at which the latest complex bitvalue of the k complex bit values is placed into the k-stage complexshift register and a specific modulo-M number associated with thecorresponding bit sync pulse.
 3. The method of claim 2, wherein saidstep (c2) for obtaining the weighted sum includes: performing a k-pointfast Fourier transform (FTT) on the weighted complex bit values togenerate k FTT outputs; calculating a magnitude of each of the k FTToutputs; and obtaining a maximum value of the magnitudes as themagnitude of the weighted sum.
 4. The method of claim 2, wherein M is300, the target segment has the known sequence of k bits at the knowndata bit location in each subframe of the navigation message, and eachbit sync pulse is tagged by a modulo-300 number, and wherein (d2)determining the data bit location includes: (e2) calculating a magnitudevalue of the weighted sum and storing the magnitude in one of aplurality of accumulators which is associated with the modulo-300 numberof the corresponding bit sync pulse at which the latest complex bitvalue is placed into the k-stage shift register; (f2) iterating steps(a2) through (d2) until the magnitude value of the weighted sums areplaced in a predetermined number of the accumulators; (g2) repeatingstep (f2) a plurality of times such that each of the predeterminednumber of accumulators accumulates a certain number of the magnitudevalues of the weighted sums; (h2) determining which of the accumulatorscontains a largest accumulated value of the magnitude values and aspecific modulo-300 number associated therewith; and (i2) identifying adata bit location of the target segment within the subframe based on thespecific modulo-300 number associated with the one of the accumulators.5. The method of claim 2, wherein M is 1500, the target segment has theknown segment of k bits at the known data bit location in each frame ofthe navigation message, and each bit sync pulse is tagged by amodulo-1500 number, and wherein (d2) determining the data bit locationincludes: (e2) determining if the magnitude of the weighted sum exceedsa predetermined threshold value; (f2) if the magnitude of the weightedsum does not exceed the predetermined threshold value, repeating steps(a2) through (d2); and (g2) if the magnitude of the weighted sum exceedsthe predetermined threshold value, identifying a data bit location ofthe target segment based on a specific modulo-1500 number of the bitsync pulse at which the latest bit value is placed into the k-stagecomplex shift register.
 6. The method of according to claim 2, whereinthe target segment is a preamble of the subframe in the navigationmessage, where number k is 8, a Time of Week (TOW) in the navigationmessage, where number k is 17, or an ephameris data segment in thenavigation message, where number k is 128 or more.
 7. The methodaccording to claim 1, further comprising: Doppler-compensating thebaseband signal before said cross-correlating, using a frequency-lockedloop (FLL).
 8. The method of claim 1, further comprising: generating afrequency error signal based on a frequency associated with the k-pointFTT output having the maximum magnitude.
 9. The method according toclaim 1, wherein the GPS/GNSS signal level is less than about −151 dBm.10. The method according to claim 1, wherein the certain time ambiguityis a 6 (±3) second ambiguity.
 11. The method according to claim 1,wherein the certain time ambiguity is a 30 (±15) second ambiguity wherewhen the target segment occurs once in every frame, and the certain timeambiguity is a 12.5 (±6.25) minute ambiguity where the target segmentoccurs once in every navigation message.
 12. A GPS/GNSS receiver havinga sub-microsecond time transfer circuit using a weak GPS/GNSS signalfrom a satellite, a complex digitized baseband signal of the GPS/GNSSsignal carrying a received PN code having a code period and a navigationmessage having data bits, each data bit corresponding to 20 codeperiods, the GPS/GNSS receiver comprising: a PN code generatorconfigured to generate a generated PN code for the satellite, the PNcode generator outputting a timing signal indicating code epochs of thegenerated PN code, each code epoch marking a beginning of each periodand being tagged with a modueo-20 number; a delay-locked loop configuredto keep the generated PN code time-aligned with the received PN code ofthe baseband signal; a cross-correlator configured to cross-correlatethe generated PN code and the baseband signal controlled by the timingsignal so as to output a complex correlation value at each code epoch, asequence of the complex correlation values forming a navication datastream representing the navigation message; a bit synchronizer coupledto the cross-correlator, and configured to locate data bit boundaries ofthe navigation data stream and generate bit sync pulses at the data bitboundaries, each of the bit sync pulses being tagged with a modulo-Mnumber; a target segment locator coupled to the bit synchronizer and thecross-correlator, and configured to detect a target segment in thenavigation message, the target segment having a known sequence of k bitsat a known data bit location in the navigation message; and atransmission time determiner configured to determine transmission timeof the target segment with a certain time ambiguity, based on thedetermined data bit location of the target segment and knowntransmission time of a particular data bit in the navigation message, tosolve the certain time ambiguity using approximate time obtained from asource external to the receiver, and to correct a propagation delay ofthe transmission time so as to obtain sub-microsecond time transfer. 13.The GPS/GNSS receiver according to claim 12, wherein the bitsynchronizer comprises: a 20-stage shift register configured to placethe complex correlation values at a timing of the corresponding codeepochs, the 20-stage shift register holding a sequence of most recent 20complex correlation values; an adder configured to sum the 20 complexcorrelation values held in the shift register at each epoch; a magnitudecalculator configured to calculate a magnitude value of the sum of the20 complex correlation values; a bank of accumulators configured tostore and accumulate the magnitude values in association with themodulo-20 number of the code epoch at which the latest complexcorrelation value of the 20 complex correlation values is placed intothe 20-stage shift register; a controller configured to determine one ofthe 20 accumulators that contains a largest accumulated value of themagnitude values and a specific modulo-20 number associated therewith,and to identify the code epoch having the specific modulo-20 number as adata bit boundary of the navigation data stream; and a bit sync pulsegenerator configured to output the bit sync pulses at a timing of thecode epochs tagged with the specific modulo-20 number, each bit syncpulse being gagged with a modulo-M number.
 14. The GPS/GNSS receiveraccording to claim 13, wherein the bank of accumulators includes 300accumulators.
 15. The GPS/GNSS receiver according to claim 12, whereinthe target segment locator comprises: a one-bit accumulator configuredto sum the complex correlation values for one data bit period inaccordance with the bit sync pulse to obtain a complex bit value foreach data bit; a k-stage shift register configured to place the complexbit value at a timing of the corresponding bit sync pulse, the k-stageshift register holding a sequence of most recent k complex bit values; aweighted multiplexer configured to obtain a weighted sum of the sequenceof the most recent k complex bit values, each of the k complex bitvalues being weighted by corresponding one of a sequence of k weights,the sequence of k weights corresponding to the known k bit sequence ofthe target segment; and a controller configured to determine the databit location of the target segment based on the weighted sum and thecorresponding bit sync pulse at which the latest complex bit value ofthe k complex bit values is placed into the k-stage shift register and aspecific modulo-M number associated with the corresponding bit syncpulse.
 16. The GPS/GNSS receiver of claim 15, wherein the target segmenthas the known sequence of k bits at the known data bit location in eachsubframe of the navigation message, M is 300, and each bit sync pulse istagged by a modulo-300 number, and wherein the target segment locatorfurther includes: a magnitude calculator coupled to the weightedmultiplexer and configured to calculate a magnitude value of eachweighted sum; and a bank of accumulators configured to store andaccumulate the magnitude value in association with the modulo-300 numbercorresponding to that of the bit sync pulse at which the latest complexbit value is place into the k-stage shift register, wherein thecontroller is configured to determine one of the accumulators whichcontains a largest value of the accumulated magnitude values and aspecific modulo-300 number associated therewith, and identify a data bitlocation of the target segment within the subframe based on the specificmodulo-300 number associated with the one of the accumulators.
 17. TheGPS/GNSS receiver of claim 15, wherein the target segment has the knownsegment of k bits at the known data bit location in each frame, whereinM is 1500, the bit sync pulse is tagged with a modulo-1500 number, andthe target segment locator further including: a comparator configured todetermining if the magnitude of the weighted sum exceeds a predeterminedthreshold value, wherein the controller is further configured toidentify, if the magnitude of the weighted sum exceeds a predeterminedthreshold value, a data bit location of the target segment based on aspecific modulo-1500 number of the bit sync pulse at which the latestbit value is placed into the k-stage shift register.
 18. The GPS/GNSSreceiver according to claim 13, wherein the GPS/GNSS signal level isless than about −151 dBm.